#include <fstream>
#include <vector>

#define DIM 100010

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

int n, m, solution;

vector<int> L[DIM];

pair<int, int> edges[DIM];

bool vis[DIM];

void DFS(int node) {

    vis[node] = true;

    for (int i = 0; i < L[node].size(); i++) {

        int child = L[node][i];

        if (vis[child])
            continue;

        DFS(child);
        edges[++solution] = make_pair(node, child);

    }

}

int main() {

    fin >> n >> m;

    for (int i = 1; i <= m; i++) {

        int x, y;

        fin >> x >> y;

        L[x].push_back(y);
        L[y].push_back(x);

    }

    DFS(1);

    if (solution != n - 1) {

        fout << "-1\n";

        return 0;

    }

    fout << 2 * solution << "\n";

    for (int i = 1; i <= solution; i++)
        fout << edges[i].second << " " << edges[i].first << "\n";

    for (int i = solution; i >= 1; i--)
        fout << edges[i].first << " " << edges[i].second << "\n";


    return 0;

}